For a liquid pump driven by an electric motor, a water tight seal is required between the motor and the pump to prevent water from leaking into the motor from the pump. The seal may be provided in conjunction with the coupling structure between the motor and the pump, where the shaft of the motor is also isolated from the liquid of the pump, as in the case where the liquid being pumped is caustic or otherwise likely to damage the shaft. A prior art pump, shown in FIGS. 6 & 7, is driven by an electric motor and includes an impeller 10, a carrier rod 31, a macerator 20 and a seal assembly. The seal assembly includes a rotating seal 32, a stationary seal 33, a rotating ring 34 and a stationary ring 35. In this design, an axial straight line knurl 43 is formed on the shaft 40. The carrier rod 31 is pressed onto the shaft 40 and held fast by the knurl so as to rotate with the shaft 40. The rotating seal 32 is press-fitted on the carrier rod 31 and is rotatable relative to the stationary seal 33 fixed to a bracket of the motor. The above construction can avoid rotational movement between the shaft 40 and the carrier rod 31. However, axial movement or separation between the shaft 40 and the carrier rod 31 may occur. Such movement of the carrier rod affects the alignment of the impeller and leads to failure of the pump. Furthermore, movement between the rotating seal 32 and the carrier rod 31 may occur, which may allow water to enter into the motor via the space between the rotating seal 32 and the carrier rod 31.
The present invention aims to provide a new coupling structure which can solve the problem of axial movement between the carrier rod and the motor shaft. In addition or alternatively, certain embodiments of the invention address the problem of movement between the rotating seal and the carrier rod.